1. Field of the Invention
The present invention relates to apparatus and methods for improving signal acquisition and measurement by satellite positioning system receivers, and more particularly to acquiring and performing measurements on low signal to noise ratio (SNR) satellite signals, as well as to reducing acquisition times for satellite signals having normal received SNRs.
2. Description of Related Art
Satellite positioning system (SATPS) receivers such as Global Positioning System (GPS) and Global Orbiting Navigational System (GLONASS) receivers, make precise determinations of latitude, longitude, elevation and time by using time difference of arrival and Doppler measurement techniques on precisely-timed spread spectrum signals transmitted by orbiting satellites. The transmitted signals contain a number of components designed to enable the receivers to extract the requisite information. In GPS, each satellite transmits two spread spectrum carrier signals centered around separate frequencies. The L1 signal is centered about a frequency of 1575.42 MHZ, and is modulated with, among other things, the coarse/acquisition (C/A) pseudo-random noise (PRN) code and the precision (P)PRN code. The C/A-code has a 1.023 MHZ chip rate, and the P code has a 10.23 MHZ chip rate. The L2 signal is centered around a frequency of 1227.6 MHZ and, at present, the only PRN code it carries it the P-code. The PRN codes are different for each of the GPS satellites deployed and allow use of a plurality of GPS satellite signals transmitted at the same frequency. In contrast, each GLONASS satellite transmits using a carrier frequency uniquely assigned to that satellite.
Different types of PRN codes are used for different system applications. For example, within the GPS system, C/A code is used for low cost, less accurate commercial applications, and P-code is used for higher accuracy military applications. Each GPS satellite is assigned PRN codes unique to that satellite. In the case of GLONASS, all satellites use the same PRN code(s).
For each SATPS satellite, the transmitted signal comprises the carrier modulated with low frequency (50 Hz, i.e., 20 msec period, for GPS) digital data bits, which include information such as the satellite's ephemeris (i.e., position), current time of day, and system status information, and further modulated with one or more PRN codes.
The typical SATPS receiver receives a composite signal consisting of one or more of the signals transmitted by the satellites within view, that is within a direct line-of-sight, as well as noise and any interfering signals. Because the signal transmitted by each satellite uses a PRN code or a carrier frequency unique to that satellite, the receiver may separate the signals from different satellites using code division multiple access (e.g., each GPS satellite has a unique PRN code) or frequency division multiple access (e.g., each GLONASS satellite has a unique carrier frequency) techniques. The composite signal is first fed to a down-converter which amplifies and filters the incoming composite signal, mixes it with a locally generated carrier reference signal, and thus produces a composite intermediate frequency (IF) signal. For a GPS receiver, a decoder or channel circuit then correlates the composite signal by multiplying it by a locally generated version of the PRN code signal assigned to a particular satellite of interest. If the locally generated PRN code signal is properly timed, the digital data from that particular satellite is then properly detected.
The PRN codes also provide a mechanism to precisely determine the signal transmission time from each satellite. By determining the transmission time from at least four satellites, and knowing each satellite's ephemeris and approximate time of day information, the receiver's three dimensional position, velocity and precise time of day may be calculated.
For more information on the format of the GPS system signals, see “Interface Control Document ICD-GPS-200, Sep. 26, 1984”, published by Rockwell International Corporation, Satellite Systems Division, Downey, Calif. 90241.
For more information on the format of the GLONASS system signals, see “The GLONASS System Technical Characteristics and Performance,@ Working Paper, Special Committee on Future Air Navigation Systems (FANS), International Civil Aviation Organization (ICAO), Fourth Meeting, Montreal, Quebec, Canada, May 2-20, 1988.
There are many practical applications of SATPS receivers in which low SNR and fast acquisition are desirable, including, for example, operation of GPS receivers indoors, where the SNR available for acquisition and measurements may be substantially below normal GPS minimum received power levels. As another example, when a GPS receiver has been powered down for a significant length of time, knowledge of oscillator offset and time may be degraded. Fast acquisition improvement would allow the receiver to respond quickly after power-on with a position fix. Low SNR militates against fast acquisition. Since the search for low satellites under low SNR involves longer integration times, each search bin must be serviced rapidly in order to have reasonable response time.
Some attempts to achieve fast acquisition under low SNR are disclosed in the prior art. U.S. Pat. No. 5,663,734 describes a method for storing a one-second snapshot of IF samples in a memory device. The samples are subsequently processed in the frequency domain by a high speed digital signal processor to reduce acquisition time under low SNR.
U.S. Pat. No. 5,420,593 issued to M. Niles and assigned to Trimble Navigation Ltd. describes an apparatus and method for more rapidly acquiring GPS signals by storing digital samples in a memory device and reading them back into a correlator device at a clock rate substantially higher than the sampling rate.